Fluids used during drilling operations include “drilling,” “drill-in,” and “completion” fluids. A “drill-in” fluid is pumped through the drill pipe while drilling through the “payzone,” or the zone believed to hold recoverable oil or gas. A “drilling fluid” is used to drill a borehole through the earth to reach the payzone. Typically a drilling mud is circulated down through the drill pipe, out the drill bit, and back up to the surface through the annulus between the drill pipe and the borehole wall. The drilling fluid has a number of purposes, including cooling and lubricating the bit, carrying the cuttings from the hole to the surface, and exerting a hydrostatic pressure against the borehole wall to prevent the flow of fluids from the surrounding formation into the borehole. A “completion fluid” is used to protect the “payzone” during the completion phase of the well.
Fluids in which water is the continuous phase provide for a fast drilling rate, and are ecologically favored over fluids in which oil is the continuous phase. Unfortunately, the walls of a wellbore frequently are composed at least in part of shale, and when exposed to water, many shales swell, slough, or spall to the extent that they may even prevent further operation of the wellbore. Shale also may slough off during gravel transport in open-hole completion, mix with the gravel, and reduce the productivity of the well by choking off the permeability of the gravel pack. The sloughing also may cause screen blinding.
Brines have long been used in the formulation of drilling fluids to take advantage of their density and their inhibitive characteristics. Clay chemistry has shown us that cationic base exchange with the negatively charged clay minerals commonly found in shale formations, limits their ability to hydrate, soften, and swell, thereby rendering them more stable in the presence of water based fluids.
Monovalent salts, such as NaCl or KCl, have long been used as make up water for drilling fluids. In the past, NaCl or KCl have provided a limited inhibitive environment for drilling hydratable shales in many areas. In offshore drilling, seawater—a complex mixture of various salts which is readily available—has frequently been used in formulating drilling mud.
Today, technological advances in the design of drilling equipment has resulted in increased penetration rates for better drilling economics. The performance of the monovalent salt systems has not been able to maintain pace with new advances in drilling technology. The need for improved drilling mud systems saw the application of oil mud systems and development of synthetic systems to meet these challenges. However, increasing environmental regulation has limited the application of these systems.
Multivalent salts, preferably divalent salts, are known to provide an added inhibitive benefit for drilling water sensitive shales. Multivalent salts are capable of developing a strong bond with and between active clay platelets in these shales, thereby further limiting the volume of hydration water that can become a part of the clay, causing it to become soft, pliable, and sticky, resulting in problems with mechanical drilling equipment and drilling fluid control. One of the most available and economical multivalent salt systems is CaCl2. CaCl2 systems have been around for many years, and the inhibitive characteristics of the calcium ion are well known.
Water-soluble polymers are used to thicken water-base fluids, and in part to synergistically stabilize shale. In drilling fluids, the water-soluble polymers provide the viscosity necessary to lift drilled solids from the wellbore, and tend to improve drilling rates.
Unfortunately, drilling fluids which have shown promise in increasing the rate of penetration through shales also have tended to increase screen blinding, which can lead to huge losses of mud, with loss of rig time and high fluid costs. Water base fluids are needed which can achieve a high rate of penetration without screen blinding.